Sickle cell disease is characterized by intermittent, localized microvascular occlusive crises. Hemoglobin-S deoxygenation leads to morphologic sickling and the formation of rigid erythrocytes which occlude capillaries and obstruct passage of subsequent red cells. Hemoglobin polymerization kinetics are such that the majority of erythrocytes traverse the microcirculation prior to morphologic sickling. However, delayed microvascular transit will increase the likelihood of intracapillary sickling, microvascular occlusion, and pain crisis. We hypothesize that erythrocyte adherence to vascular endothelium in the microcirculation delays erythrocyte transit sufficiently to initiate or propagate sickle cell vaso-occlusion. Erythrocyte/endothelial adherence is likely related to complex changes in the erythrocyte surface, the endothelium, and the vascular milieu. In vitro, sickle red cell adherence quantitatively correlates with the clinical severity of sickle cell disease. Furthermore, acute phase reactants, such as fibrinogen and high molecular weight vWF multimers as well as factors released from activated platelets, promote sickle red cell adherence. In addition, adhesion to large vessel and microvascular endothelium is qualitatively and quantitatively different. We propose to investigate the mechanism of adherence to venous, arterial, and microvascular endothelium during crisis and asymptomatic periods in order to identify plasma components and cell adhesion molecules which promote sickle erythrocyte adherence to cultured endothelium. It is also proposed to investigate the role of endothelial cell stimulation which may occur as a result of activation of coagulation, infection, or pregnancy in sickle patients. Endothelial cell activation would lead to modulation of cell adhesion molecule expression and/or secretion of adhesive proteins. In the proposed experiments, a dynamic in vitro adherence assay which incorporates the fluid shear forces present in the post-capillary venules in vivo will be utilized. Using this system, we will (i) characterize the difference in adhesion of sickle erythrocytes to endothelial cells from microvessels and umbilical vein by determining the effects of agonists and antagonists on adherence, (ii) determine the variability in sickle red cell adherence to endothelial cells from patient to patient and for individual patients during crisis and asymptomatic periods, (iii) identify plasma factors, including soluble factors released from activated platelets, which promote sickle erythrocyte adherence, and (iv) contrast the effects of agonists which presumably stimulate endothelial cells, such as thrombin, endotoxin, plasmin, fibrin(ogen), and TNF on adhesive protein secretion, cell adhesion molecule expression, and adherence.